I'd like to add that two-part catalyzed putty is great. I prefer Evercoat Euro Soft glazing putty. It is very fine grained. It seems to work best over a lacquer primer. Many "tube" type putties cure by a solvent evaporating out, allowing it to shrink and crack.
Look for a book on customizing model cars by Mark Gustavson. He also had a series of articles called "Custom Clinic", In Car Modeler magazine. He tells how to do custom body work like chop tops, sectioning, channeling, etc. Well worth it to find the magazines and/or the book.